Environmental conditions in winter and their ecological and evolutionary consequences for American redstarts (Setophaga ruticilla)

Abstract

I used both observational and experimental approaches to assess the causes of nonbreeding habitat quality and to evaluate their ecological and evolutionary consequences for a Neotropical-Nearctic migratory bird, the American redstart (Setophaga ruticilla). Relative to control birds overwintering in second-growth scrub, redstarts experimentally upgraded from scrub to mangrove forest incorporated mangrove stable-carbon isotope signatures, maintained mass over winter, departed earlier on spring migration, and had higher apparent annual survival. Significantly higher arthropod biomass on upgrade territories implicated food availability as a proximate mechanism of habitat quality. Food availability, body condition, and spring departure schedules also depended on nonbreeding season rainfall. Food availability in mangrove was higher than in scrub in three of four years, allowing birds in this habitat to maintain superior body condition and depart earlier on spring migration. Abundant rainfall in a single year led to abnormally high food availability in scrub and early departure of birds in both habitats, suggesting both the amount and timing of rainfall influenced nonbreeding performance. Habitat occupancy and annual variation in rainfall had significant consequences for natal dispersal and selection through annual survival. Stable-hydrogen isotope ratios (δD) in feathers of immature birds captured again as adults indicated that habitat use in the first nonbreeding season interacted with spring phenology on temperate breeding grounds to influence the distance traveled on the first spring migration and direction of natal dispersal. In contrast, adults showed site fidelity between breeding seasons, suggesting nonbreeding conditions did not affect breeding dispersal and that migration distance becomes fixed later in life. Patterns of δD also revealed directional selection for short migration distance to southern breeding areas, a pattern that was nearly twice as strong in scrub compared to mangrove. During dry winters, redstarts experienced stabilizing selection on departure dates and directional selection for short migration. In years of high rainfall, birds experienced correlational selection favoring late departure when in good body condition. Thus, occupancy of moist habitats and years of high rainfall relaxed selection against late departure and longer migration. Collectively, these findings emphasize the need to understand how events throughout the annual cycle interact to shape fundamental biological processes.